LTC3714_15 Datasheet, PDF (16/28 Pages) Linear Technology – Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp

LTC3714
Applications Information
Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3714, the maximum sense voltage is controlled
by the voltage on the V_RNG pin. With valley current control,
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
I_LIMIT
=
V_{SNS(MAX )}
*R_DS(ON)ρ_T
+
1
2
ΔI_L
The current limit value should be checked to ensure that
I_LIMIT(MIN) > I_OUT(MAX). The minimum value of current
limit generally occurs with the lowest V_IN at the highest
ambient temperature. Note that it is important to check
for self-consistency between the assumed junction tem-
perature and the resulting value of I_LIMIT which heats the
MOSFET switches.
Caution should be used when setting the current limit based
upon the R_DS(ON) of the MOSFETs. The maximum current
limit is determined by the minimum MOSFET on-resistance.
Data sheets typically specify nominal and maximum values
for R_DS(ON), but not a minimum. A reasonable assumption
is that the minimum R_DS(ON) lies the same amount below
the typical value as the maximum lies above it. Consult the
MOSFET manufacturer for further guidelines.
To further limit current in the event of a short-circuit to
ground, the LTC3714 includes foldback current limiting.
If the output falls by more than 50%, then the maximum
sense voltage is progressively lowered to about one sixth
of its full value.
Minimum Off-Time and Dropout Operation
The minimum off-time t_OFF(MIN) is the smallest amount of
time that the LTC3714 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 250ns.
The minimum off-time limit imposes a maximum duty
cycle of t_ON/(t_ON + t_OFF(MIN)). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
V_IN(MIN)
=
V_OUT
t_ON
+ t_OFF(MIN)
t_ON
Output Voltage Programming
The output voltage is digitally set to levels between 0.6V
and 1.75V using the voltage identification (VID) inputs
VID0-VID4. An internal 5-bit DAC configured as a precision
resistive voltage divider sets the output voltage in incre-
ments according to Table 1. The VID codes are compatible
with Intel Mobile Pentium^® III processor specifications.
Each VID input is pulled up by an internal 2.5µA current
source from the INTV_CC supply and includes a series diode
to prevent damage from VID inputs that exceed the supply.
*Use R_SENSE value here if a sense resistor is connected between SENSE and PGND.
3714f
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